Dielectric ceramic composition and dielectric resonator

ABSTRACT

A dielectric ceramic composition possessing a high relative dielectric constant, a high unloaded Q, and an excellent temperature characteristic is expressed as xPbO-yZrO 2  -zCaO, in the relation of x+y+z=1, wherein x, y, z are in mol percentage ranges of 0.125≦x≦0.49, 0.4≦y≦0.6, 0.11 ≦z≦0.275. A part of ZrO 2  of the composition formula may be replaced by (B1B2)0 2  (where B1 is Mg, Ca, Zn, Co, Ni, Mn, Cu, Al, Ga, or Y, and B2 is Nb, Ta, or W), or Sn.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dielectric ceramic used in the microwave region, and a dielectric resonator using this ceramic.

2. Description of the Prior Art

Recently, as communication using electromagnetic waves in the microwave region has advanced, such as mobile telephones portable telephones and satellite broadcasting, it is demanded to downsize the equipment. For this purpose, it is necessary to reduce the size of the individual components composing the equipment. The dielectric is incorporated in such devices as a dielectric resonator in the filter element or oscillator. The size of the dielectric resonator is inversely proportional to the square root of the dielectric constant of the dielectric material itself when using the same resonance mode, and therefore manufacture of a small-sized dielectric resonator requires a material of high dielectric constant. Besides, other requirements of the dielectric include low loss in the microwave region or the high unloaded Q value, small changes of the resonant frequency with temperature, or small changes of the dielectric constant depending on temperature.

Dielectric materials hitherto used in this field include, among others, Ba(Zn_(1/3) Ta_(2/3))O₃, BaO-TiO₂ compositions, and their derivatives having some of the elements replaced by other elements. In all these materials, the relative dielectric constant is about 30, and it is too low to downsize the resonator. As a material with a higher dielectric constant, BaO-TiO₂ -Sm₂ O₃ composition was disclosed in U.S. Pat. No. 4,330,631 (May 18, 1982). This material has a relative dielectric constant of about 80, a high unloaded Q of about 3000 at 2 to 4 GHz, and a small temperature coefficient of the relative dielectric constant. In addition, BaO-PbO-TiO₂ -Nd₂ O₃ is known to have a relative dielectric constant of over 90, as reported in the Journal of American Ceramic Society, Vol 67 (1984), pp.278-281.

However, for further reduction of the resonator size, materials of much higher dielectric constant are demanded, but materials possessing both high unloaded Q and small temperature coefficient are not known yet so far. On the other hand, the value of the temperature coefficient of the dielectric constant is generally negative when the dielectric constant is higher, but some materials have a positive value such as PbZrO₃. It is hence attempted to reduce the temperature coefficient by combining materials having positive and negative temperature coefficient values. Materials having negative temperature coefficient include TiO₂ and SrTiO₃, among others, and materials having positive temperature coefficient are represented, for example, by PbO-ZrO₂ -Tb₄ O₇ which is disclosed in U.S. Pat. No. 4,849,384 (Jul. 18, 1989), as the system having lanthanide oxide added to PbO-ZrO₂. This material has the relative dielectric constant of over 100, and contributes greatly to downsizing of the resonator, but the temperature coefficient of the resonant frequency, τ_(f), is about -1,000 ppm/°C., which shows a large change by temperature.

SUMMARY OF THE INVENTION

It is hence a first object of the invention to present a material having a relative dielectric constant larger than that of the hitherto known material BaO-TiO₂ -Sm₂ O₃, a low loss in a microwave region, and a small temperature coefficient of the dielectric constant.

It is a second object of the invention to present a dielectric resonator smaller in size than the dielectric resonator using any hitherto known material.

To achieve these objects, the invention presents a novel composition expressed as xPbO-yZrO₂ -zCaO, where x+y+z=1, and x, y, z are in mol percentage ranges of 0.125≦x≦0.49, 0.4≦y ≦0.6, 0.11≦z≦0.275, respectively. This novel composition possesses a high specific dielectric constant of over 40 in a microwave region of 2 to 6 GHz, and also a small temperature coefficient not exceeding 300 ppm/°C., and a high unloaded Q value of over 300.

The invention is further intended to improve the dielectric characteristics in the microwave region by replacing a part of the ZrO₂ of the formula expressed by xPbO-yZrO₂ -zCaO by (B1B2)O₂ (where B1 is one of Mg, Ca, Zn, Co, Ni Mn, Cu, Al, Ga, Y, and B2 is one of Nb, Ta, W), or Sn.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a part of the ternary system indicating a desired composition range of xPbO-yZrO₂ -zCaO, in which sample numbers correspond to Table 1.

FIG. 2 is a composition diagram showing a desired composition range of (Pb_(1-x) Ca_(x))Zr_(1-y) (Mg_(1/3) Nb_(2/3))_(y) O₃ system, in which sample numbers correspond to Table 3.

FIG. 3 is a composition diagram showing a desired composition range of (Pb_(1-x) Ca_(x))Zr_(1-y) (Ni_(1/3) Nb_(2/3))_(y) O₃ system, in which sample numbers correspond to Table 4.

FIG. 4 is a composition diagram showing a desired composition range of (Pb_(1-x) Ca_(x))Zr_(1-y) Sn_(y) O₃ system, in which sample numbers correspond to Table 5.

FIG. 5a is a diagram showing a longitudinal sectional view and FIG. 5b is a diagram showing a lateral sectional view of a coaxial resonator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

Using PbO, CaCO₃, ZrO₂ as starting materials, they were weighed as specified according to x, y, z of the formula xPbO-yZrO₂ -zCaO so as to achieve the composition shown in Table 1. These powder materials were put in a polyethylene ball mill containing stabilized zirconia pebbles, and purified water was added to the mixture, and mixed for 17 hours. After mixing, the slurry was dried, and put in an alumina crucible, and was calcined for 2 hours at 750° to 850° C. The calcined material was crushed by pulverizer, and ground in the same ball mill for 17 hours, and dried and material powder was obtained. To this powder, 5% aqueous solution of polyvinyl alcohol was added as binder by 6 wt. %, and mixed, and the mixture was passed through a 32-mesh sieve and granulated, and pressed and formed into a column of 13 mm in diameter and about 5 mm in thickness at 100 MPa. This forming was heated to 600° C. for 2 hours to burn out the binder, and the residue was put into a magnesia container, and sintered for 2 hours at 1,100° to 1,400° C. depending on the composition while preventing evaporation of PbO by spreading the calcined powder of the same composition in the surroundings. The obtained sinter was measured by the dielectric resonator method, and the resonant frequency and unloaded Q were determined. Besides, from the dimensions of the sinter and the resonant frequency, the relative dielectric constant was calculated. The resonant frequency was 2 to 6 GHz. At -25° C. and 85° C., the resonant frequency was measured, and the temperature coefficient (τ_(f)) was calculated on the basis of the value at 20° C. The results are shown in Table 1.

As clear from Table 1, in the composition in which x, y, z are respectively in the ranges of 0.125≦x≦0.49, 0.4≦y ≦0.6, 0.11≦z≦0.275, the dielectric constant is as high as 40 to 165, and the unloaded Q is over 200, and the temperature coefficient of the resonant frequency does not exceed 150 ppm/°C. In particular, the sample numbers 5, 7, 11, 12 in which x, y, z are in the ranges of 0.28≦x≦0.425, 0.45≦y ≦0.52, 0.125≦z≦0.2 present excellent characteristics with the relative dielectric constant of over 100, unloaded Q of over 300, and temperature coefficient of resonant frequency of 40 ppm/°C. or less.

However, the compositions not included in the above range, that is, the range in FIG. 1 bounded by sample numbers 15, 14, 19 and 20, are not suitable as dielectrics for microwave devices. More specifically, in the compositions of which CaO is less than 0.11 (sample numbers 1, 2, 3, 10, 17 in Table 1), the unloaded Q is less than 200, or the temperature coefficient of the resonant frequency is over 200 ppm/°C., or both, and they are not applicable as dielectric resonators. Or in the composition of which CaO is more than 0.275 (sample number 9 in Table 1), the relative dielectric constant is less than 40. If ZrO₂ is less than 0.4 (sample 16 in Table 1), the unloaded Q is less than 200, and in the composition exceeding 0.6 (sample number 21 in Table 1), the dielectric is not sintered densely at temperature of less than 1,400° C.

Embodiment 2

In addition to the starting materials of Embodiment 1, powders of MgO, ZnO, CoO, NiO, MnO₂, CuO, Al₂ O₃, Ga₂ O₃, Y₂ O₃, Nb₂ O₅, Ta₂ O₅, Sb₂ O₃, WO₃ were weighed as specified so as to compose as Shown in Table 2, according to x, y, z, m, and n of the composition formula, Pb_(1-x) Ca_(x) Zr_(1-y) (B1_(m) B2_(n))_(y) O₃ (where B1 is an element selected from a group of Mg, Ca, Zn, Co, Ni, Mn, Cu, Al, Ga, and Y, B2 is an element selected from a group of Nb, Ta, Sb, and W, and m and n are the values selected to satisfy the relation of axm+bxn=4, while keeping the relation of m+n=1, supposing the valence of element B1 as a, and the valence of element B2 as b), and ceramics were prepared in the same manner as in Embodiment 1, and the dielectric characteristics were measured, of which results are shown in Table 2.

As clear from Table 2, in the compositions in which x and y of the formula (Pb_(1-x) Ca_(x))Zr_(1-y) (B1_(m) B2_(n))_(y) O₃ are in the ranges of 0.25≦x≦0.5, and 0≦y≦0.2, respectively, the dielectric constant is 60 or higher, the unloaded Q is 300 or higher, and the absolute value of temperature coefficient of resonant frequency is 100 ppm/°C. or less. Besides, in the compositions of sample numbers 1 to 5 in which Zr is not replaced by (B1_(m) B_(n)) the results of two sintering processes did not match, but in the compositions having Zr replaced by (B1_(m) B2_(n)) (sample numbers 18, 33, 36, 38, 40, 41, 42, 47), the sintering temperature is 1,250° C. or less, and the results of the sintering test performed twice matched well. This is considered due to decrease of the sintering temperature, small evaporation of PbO, and hence the characteristics are stabilized.

Incidentally, where B1 is Mg, B2 is Nb, m is 1/3, and n is 2/3, that is, when ZrO₂ of PbO-ZrO₂ -CaO is replaced by (Mg_(1/3) Nb_(2/3))O₂, an excellent dielectric characteristic was obtained in the microwave region, for the substitutional volume in a wide range where x exceeds 0.25 in the above formula. The results are shown in Table 3.

In Table 3, the composition of which dielectric constant is as high as 64 to 151, unloaded Q is 200 or higher, and absolute value of temperature coefficient resonant frequency is 150 ppm/°C. or less are in a range of a polygon in which x and y as expressed in the formula (Pb_(1-x) Ca_(x))Zr_(1-y) (Mg_(1/3) Nb_(2/3))_(y) O₃ are on an xy plane shown in FIG. 2, having the vertexes at the following points, A (x=0.35 y=1.0), B (x=0.6 y=1.0), C (x=0.6 y=0.25, D (x=0.5 y=0.0), E (x=0.25 y=0.0), F (x=0.45 y=0.5). However, in the compositions in which x and y are not included in the polygon ABCDEF, any one of the following is present, in which the dielectric constant is 50 or less, the unloaded Q is 200 or less, and the temperature coefficient of the resonant frequency is 200 ppm/°C. or more, and desired characteristics as the dielectric for microwave are not obtained.

Furthermore, when B1 is Ni, B2 is Nb, m is 1/3, and n is 2/3, that is, when ZrO₂ of PbO-ZrO₂ -CaO is replaced by (Ni_(1/3) Nb_(2/3))O₂, same as when replaced by (Mg_(1/3) Nb_(2/3)), excellent dielectric characteristics were obtained in the microwave region, for substitutional volume in a wide range. The results are shown in Table 4.

In Table 4, the compositions of which dielectric constant is as high as 68 to 172, unloaded Q is 250 or more, and absolute value of temperature coefficient of resonant frequency is 100 ppm/°C. or less are within a range of a quadrangle in which x and y as expressed in the formula (Pb_(1-x) -Ca_(x)) Zr_(1-y) (Ni_(1/3) Nb_(2/3))_(y) O₃ are on an xy plane shown in FIG. 3, with the vertexes at A (x=0.3 y=0.5), B (x=0.6 y=0.5), C (x=0.3 y=0.0), and D (x=0.5 y=0.0). In particular, sample numbers 7 and 11 present superior characteristics with the relative dielectric constant of 120 or more, and absolute value of temperature change of resonant frequency of 10 ppm/°C. or less. In sample numbers 4, 14, 15, 16, the absolute values of temperature coefficient of resonant frequency are 100 ppm/°C. or less, and the unloaded Q is 1,000 or more, and excellent characteristics were exhibited. However, in the compositions not included in the quadrangle ABCD, any one of the following is present, in which the relative dielectric constant is 50 or less, the unloaded Q is 200 or less, or the temperature coefficient of resonant frequency is 200 ppm/°C. or more, and desired characteristics as the dielectric for microwaves are not obtained.

Embodiment 3

In addition to the starting materials in Embodiment 1, SnO₂ powder was used, and weighed as specified so that x and y in the formula (Pb_(1-x) Ca_(x))Zr_(1-y) Sn_(y) O₃ may form the composition as shown in Table 5, and ceramics were prepared in the same manner as in Embodiment 1. The results of measuring the dielectric characteristics of these ceramics are shown in Table 5.

As clear from Table 5, in the compositions in which x and y are in a range of a quadrangle having the following points as the vertexes on the xy plane shown in FIG. 4; A (x=0.3 y=0.0), B (x=0.5 y=0.0), C (x=0.4 y=0.6), D (x=0.25 y=0.6), the dielectric constant is 60 or higher, the unloaded Q is 700 or higher, and the absolute value of temperature coefficient of resonant frequency is 100 ppm/°C. or less. As compared with the compositions not containing Sn shown in sample numbers 1 to 4, although the unloaded Q is improved, in the compositions not included in the quadrangle ABCD, any one of the following is present, in which the dielectric constant is 60 or less, the unloaded Q is 700 or less, or the temperature coefficient of the resonant frequency is 100 ppm/° C. or more, and desired characteristics as the dielectric for microwaves are not obtained.

Embodiment 4

Of the compositions disclosed in Embodiments 1, 2 and 3, those shown in Table 6 are selected, and coaxial dielectric resonators shown in FIG. 5 were fabricated. The fabricating method of the dielectric resonators is described below. The powder combined with binder as shown in Embodiment 1 was pressed and formed into a cylindrical sample of 8.3 mm in outer diameter, 2.5 mm in inner diameter, and about 10 mm in height, and after burning out the binder, the material was sintered in the sintering method in Embodiment 1, and a cylindrical sinter 10 was fabricated. The sintered sample was coated with a silver paste on the inner wall, outer wall and one bottom of the cylindrical sinter 10 as shown in FIG. 5, and it was dried and baked at 800° C. for 15 minutes, and an electrode 20 was manufactured.

The inner diameter 2a, outer diameter 2b, height (length) L, resonant frequency, Q, and temperature coefficient τ_(f) of the resonant frequenty of thus manufactured coaxial dielectric resonators are shown in Table 6.

As shown in Table 6, the resonators using the dielectrics of the invention are smaller in size than the resonators using the materials of the highest relative dielectric constant (90) among the hitherto known dielectrics, and at the same time it is evident that the Q value and temperature characteristics are equally comparable.

Thus, the ceramic compositions of the invention contain materials having a high relative dielectric constant of over 100 in a microwave region, and these materials are also high in the unloaded Q with 1,000 or more, and are sufficiently small in the temperature coefficient practically. Therefore, the ceramic compositions of the invention are not only useful as the material for dielectric resonator, but also effective for downsizing the circuit if applied in substrates for forming microwave circuits.

                                      TABLE 1                                      __________________________________________________________________________     Composition     firing Relative  Temperature coefficient                       Sample                                                                             x   y   z   temperature                                                                           dielectric                                                                          Unloaded                                                                            of resonant freq.                             No. PbO ZrO.sub.2                                                                          CaO °C.                                                                            constant                                                                            Q    τ.sub.f ppm/°C.                    __________________________________________________________________________      1* 0.5 0.5 0   1200   125  470  -1500                                          2* 0.45                                                                               0.5 0.05                                                                               1250   134   95  -1250                                          3* 0.4 0.5 0.1 1300   195   70  -700                                           4  0.39                                                                               0.5 0.11                                                                               1300   165  280  +50                                            5   0.0375                                                                            0.5  0.125                                                                             1300   162  310  -25                                            6  0.35                                                                               0.5 0.15                                                                               1350   142  560  +65                                            7   0.325                                                                             0.5  0.175                                                                             1350   115  820  +28                                            8  0.25                                                                               0.5 0.25                                                                               1350    52  810  -40                                            9* 0.2 0.5 0.3 1400    35  750  -43                                            10*                                                                               0.45                                                                                0.45                                                                              0.1 1150   180   80  -260                                          11  0.4  0.45                                                                              0.15                                                                               1150   124  410  -35                                           12  0.35                                                                                0.45                                                                              0.2 1150   106  300  - 38                                          13   0.425                                                                             0.4  0.175                                                                             1100    85  450  -100                                          14  0.49                                                                               0.4 0.11                                                                               1100   120  200  -130                                          15   0.325                                                                             0.4  0.275                                                                             1200    66  430  -40                                            16*                                                                               0.45                                                                                0.35                                                                              0.2 1100    72  130  -42                                            17*                                                                               0.35                                                                                0.55                                                                              0.1 1350   180   45  -2000                                         18  0.3  0.55                                                                              0.15                                                                               1350   130  510  +80                                           19  0.29                                                                               0.6 0.11                                                                               1350    82  300  +50                                           20   0.125                                                                             0.6  0.275                                                                             1400    43  760  -40                                            21*                                                                               0.2  0.65                                                                              0.15                                                                               not sinter                                                                            --   --   --                                            __________________________________________________________________________      *The asterisk refers to a comparative example out of the range of the          invention.                                                               

                                      TABLE 2                                      __________________________________________________________________________     Composition          Firing Relative dielectric                                                                          Temperature coefficient              Sample                                                                             x  Chemical formula                                                                        y    temperature                                                                           constant Unloaded                                                                            of resonant freq.                    No. Pb of B1.sub.m B2.sub.n                                                                    B1.sub.m B2.sub.n                                                                   °C.                                                                            1st exp.                                                                           2nd exp.                                                                            Q    τ.sub.f ppm/°C.           __________________________________________________________________________      1  0.25                                                                              --       0    1300   162 155  310  -30                                   2  0.35                                                                              --       0    1350   115 108  820  +60                                   4  0.4                                                                               --       0    1350    91  97  860  -24                                   5  0.5                                                                               --       0    1350    52  49  810  -65                                   6  0.35                                                                              Mg.sub.1/3 Nb.sub.2/3                                                                   0.05 1350   105 111  800  +37                                   7  0.35                                                                              Mg.sub.1/3 Nb.sub.2/3                                                                   0.1  1350   124 116  770  +70                                   8  0.35                                                                              Mg.sub.1/3 Nb.sub.2/3                                                                   0.25 1350   151 155  490  +125                                  9  0.5                                                                               Mg.sub.1/3 Nb.sub.2/3                                                                   0.1  1350    63  67  1050 - 53                                 10  0.35                                                                              Ni.sub.1/3 Nb.sub.2/3                                                                   0.05 1350   108 --   1120 +35                                  11  0.35                                                                              Ni.sub.1/3 Nb.sub.2/3                                                                   0.1  1350   130 --   1050 +50                                  12  0.38                                                                              Ni.sub.1/3 Nb.sub.2/3                                                                   0.1  1400   113 --   1170 +30                                  13  0.38                                                                              Ni.sub.1/3 Nb.sub.2/3                                                                   0.15 1400   120 --   1010 +37                                  14  0.4                                                                               Ni.sub.1/3 Nb.sub.2/3                                                                   0.25 1350   134 --   330  +20                                  15  0.35                                                                              Zn.sub.1/3 Nb.sub.2/3                                                                   0.05 1300   120 --   960  +53                                  16  0.35                                                                              Co.sub.1/3 Nb.sub.2/3                                                                   0.05 1350   121 --   1050 +36                                  17  0.35                                                                              Co.sub.1/3 Nb.sub.2/3                                                                   0.1  1300   114 --   1030 +39                                  18  0.35                                                                              Cu.sub.1/3 Nb.sub.2/3                                                                   0.05 1200   102 101  770  +58                                  19  0.35                                                                              Mn.sub.1/2 Nb.sub.1/2                                                                   0.1  1350   108 --   780  +29                                  *20 0.35                                                                              Mn.sub.1/2 Nb.sub.1/2                                                                   0.25 1300    96 --   410  +32                                  21  0.35                                                                              Al.sub.1/2 Nb.sub.1/2                                                                   0.05 1350    88 --   940  +36                                  22  0.35                                                                              Y.sub.1/2 Nb.sub.1/2                                                                    0.05 1350    91 --   1020 +24                                  23  0.35                                                                              Mg.sub.1/3 Ta.sub.2/3                                                                   0.05 1400   112 118  1030 +39                                  24  0.35                                                                              Mg.sub.1/3 Ta.sub.2/3                                                                   0.1  1400   123 113  790  +51                                  25  0.35                                                                              Ca.sub.1/3 Ta.sub.2/3                                                                   0.05 1400   118 --   880  +62                                  26  0.35                                                                              Ca.sub.1/3 Ta.sub.2/3                                                                   0.1  1400   107 --   750  +56                                  27  0.35                                                                              Ni.sub.1/3 Ta.sub.2/3                                                                   0.05 1400   112 106  1180 +34                                  28  0.35                                                                              Ni.sub.1/3 Ta.sub.2/3                                                                   0.1  1400   116 106  960  +37                                  29  0.35                                                                              Zn.sub.1/3 Ta.sub.2/3                                                                   0.05 1300   121 --   940  +43                                  30  0.35                                                                              Zn.sub.1/3 Ta.sub.2/3                                                                   0.1  1300   130 --   820  +52                                  31  0.35                                                                              Co.sub.1/3 Ta.sub.2/3                                                                   0.05 1350   112 --   950  +26                                  32  0.35                                                                              Co.sub.1/3 Ta.sub.2/3                                                                   0.1  1300   127 --   890  +35                                  33  0.25                                                                              Ni.sub.1/2 W.sub.1/2                                                                    0.1  1050   191 188  390  -65                                  34  0.3                                                                               Ni.sub.1/2 W.sub. 1/2                                                                   0.05 1300   144 142  760  +77                                  35  0.35                                                                              Ni.sub.1/2 W.sub.1/2                                                                    0.05 1300   114 116  1040 +15                                  36  0.35                                                                              Ni.sub.1/2 W.sub.1/2                                                                    0.15 1100   125 124  590  -52                                  *37 0.35                                                                              Ni.sub.1/2 W.sub.1/2                                                                    0.3  1050    96  97  260  -150                                 38  0.4                                                                               Ni.sub.1/2 W.sub.1/2                                                                    0.1  1250    95  95  1130  -4                                  39  0.35                                                                              Mg.sub.1/2 W.sub.1/2                                                                    0.05 1350   115 110  910  +30                                  40  0.35                                                                              Mg.sub.1/2 W.sub.1/2                                                                    0.1  1250   117 118  800  +41                                  41  0.35                                                                              Co.sub.1/2 W.sub.1/2                                                                    0.05 1250   120 122  870  +51                                  42  0.35                                                                              Cu.sub.1/2 W.sub.1/2                                                                    0.1  1150   132 129  690  -40                                  43  0.35                                                                              Mn.sub.2/3 W.sub.1/3                                                                    0.05 1300   106 109  750  -15                                  44  0.35                                                                              Mg.sub.1/3 Sb.sub.2/3                                                                   0.05 1300   116 112  790  -55                                  45  0.35                                                                              Ni.sub.1/3 Sb.sub.2/3                                                                   0.05 1300   121 123  650  -63                                  46  0.35                                                                              Co.sub.1/3 Sb.sub.2/3                                                                   0.05 1300   107 109  630  -58                                  47  0.35                                                                              Zn.sub.1/3 Sb.sub.2/3                                                                   0.05 1250   114 111  560  -66                                  __________________________________________________________________________      *The asterisk refers to a comparative example out of the range of the          invention.                                                               

                                      TABLE 3                                      __________________________________________________________________________     Composition Firing Relative  Temperature coefficient                           Sample                                                                             x  y    temperature                                                                           dielectric                                                                          Unloaded                                                                            of resonant freq.                                 No. Ca (Mg.sub.1/3 Nb.sub.2/3)                                                             °C.                                                                            constant                                                                            Q    τ.sub.f ppm/°C.                        __________________________________________________________________________     *1  0.25                                                                              1.0  1250   323   60  +600                                              2   0.35                                                                              1.0  1200   124  310  +110                                              3   0.6                                                                               1.0  1150    97  640   +35                                              *4  0.35                                                                              0.75 1300   192  100  +305                                              5   0.5                                                                               0.75 1300   111  290  +110                                              6   0.6                                                                               0.75 1300    72  1300  +38                                              *7  0.35                                                                              0.5  1300   201   95  +200                                              8   0.45                                                                              0.5  1300   132  220  +155                                              9   0.5                                                                               0.5  1350   116  270  +115                                              10  0.6                                                                               0.5  1350    73  1280  +17                                              *11 0.7                                                                               0.5  1400    46  1550  -45                                              12  0.35                                                                              0.25 1350   151  490  +125                                              13  0.5                                                                               0.25 1400    90  530   +46                                              14  0.6                                                                               0.25 1400    64  1330  +25                                              *15 0.2                                                                               0.0  1300   195   70  +700                                              16  0.25                                                                              0.0  1300   162  310   +42                                              17  0.5                                                                               0.0  1350    52  810   +28                                              *18 0.6                                                                               0.0  1400    35  750   +30                                              __________________________________________________________________________      *The asterisk refers to a comparative example out of the range of the          invention.                                                               

                                      TABLE 4                                      __________________________________________________________________________     Composition Firing Relative  Temperature coefficient                           Sample                                                                             x  y    temperature                                                                           dielectric                                                                          Unloaded                                                                            of resonant freq.                                 No. Ca (Ni.sub.1/3 Nb.sub.2/3)                                                             °C.                                                                            constant                                                                            Q    τ.sub.f ppm/°C.                        __________________________________________________________________________     *1  0.45                                                                              0.75 1250   237   77  +360                                              *2  0.45                                                                              0.6  1300   162  320  +240                                               3  0.3                                                                               0.5  1300   118  280  +85                                                4  0.6                                                                               0.5  1350    68  1370 -90                                                5  0.45                                                                              0.35 1350   147  250  +43                                               *6  0.2                                                                               0.3  1350   115  150  +260                                               7  0.45                                                                              0.3  1350   134  250   -5                                               *8  0.75                                                                              0.3  1400    46  300  -80                                                9  0.3                                                                               0.25 1350   172  510  +100                                              10  0.4                                                                               0.25 1350   134  330  +20                                               11  0.45                                                                              0.25 1350   125  280   +5                                               12  0.4                                                                               0.2  1400   125  380  +25                                               13  0.4                                                                               0.15 1400   116  530  +30                                               14  0.35                                                                              0.1  1350   130  1050 +50                                               15  0.38                                                                              0.1  1400   113  1170 +30                                               16  0.35                                                                              0.05 1350   108  1120 +35                                               17  0.3                                                                               0.0  1350   142  560  +130                                              18  0.5                                                                               0.0  1350    52  810  -65                                               __________________________________________________________________________      *The asterisk refers to a comparative example out of the range of the          invention.                                                               

                  TABLE 5                                                          ______________________________________                                              Composi-  Firing                 Temperature                              Sam- tion      temper-  Relative      coefficient of                           ple  x      y      ature  dielectric                                                                            Unloaded                                                                              resonant freq.                         No.  Ca     Sn     °C.                                                                            constant                                                                              Q      τ.sub.f ppm/°C.             ______________________________________                                          1   0.3    0      1300   154     300   +90                                     2   0.35   0      1350   115     820   +60                                     3   0.4    0      1350    91     860   -24                                     4   0.5    0      1400    51     810   -59                                     5   0.3    0.05   1400   146     700   +95                                     6   0.35   0.05   1400   112    1190   +37                                     7   0.4    0.05   1400    94    1390   -15                                    *8   0.25   0.1    1350   145     570   +120                                    9   0.3    0.1    1400   126     930   +89                                    10   0.35   0.1    1400   107    1330   +29                                    11   0.4    0.1    1400    91    1400   -29                                    12   0.35   0.15   1400   101    1340   +17                                    13   0.4    0.15   1400    88    1340    +2                                    14   0.35   0.2    1400   100    1310   +25                                    *15  0.5    0.2    1450    46    1160   +103                                   16   0.35   0.3    1400    94    1270   +18                                    17   0.35   0.5    1400    87    1350    +9                                    18   0.25   0.6    1350    81     880   +11                                    19   0.4    0.6    1400    64    1400   -54                                    *20  0.4    0.65   1400    53    1250   +86                                    ______________________________________                                          *The asterisk refers to a comparative example out of the range of the          invention.                                                               

                                      TABLE 6                                      __________________________________________________________________________                       inner diameter                                                                         outer diameter                                                                         length                                                                             resonant                                 sample                                                                             Dielectric    (2a)    (2b)    (L) frequency                                                                               τ.sub.f                     No. composition   mm      mm      mm  GHz   Q  ppm/°C.                  __________________________________________________________________________     *1  Ba.sub..5 Pb.sub..5 Nd.sub.2 Ti.sub.5 O.sub.14                                               7.11    2.14    8.98                                                                               878   420                                                                               +55                             2   Pb.sub..65 Ca.sub..35 ZrO.sub.3                                                              6.85    2.06    7.76                                                                               881   430                                                                               +33                             3   Pb.sub..64 Ca.sub..36 ZrO.sub.3                                                              6.99    2.11    8.15                                                                               890   430                                                                               +26                             4   Pb.sub..63 Ca.sub..37 ZrO.sub.3                                                              7.00    2.11    8.20                                                                               897   450                                                                               +17                             5   Pb.sub..65 Ca.sub..35 Zr.sub..95 Sn.sub..05 O.sub.3                                          7.03    2.12    8.12                                                                               882   480                                                                               +28                             6   Pb.sub..65 Ca.sub..35 Zr.sub..925 Sn.sub..075 O.sub.3                                        7.05    2.12    8.20                                                                               884   470                                                                               +24                             7   Pb.sub..64 Ca.sub..36 Zr.sub..95 Sn.sub..05 O.sub.3                                          7.04    2.12    8.24                                                                               879   440                                                                               +20                             8   Pb.sub..62 Ca.sub..38 Zr.sub..92 (Ni.sub.1/3 Nb.sub.2/3).sub..08               O.sub.3       6.82    2.05    8.38                                                                               870   400                                                                               +21                             __________________________________________________________________________      *The asterisk refers to the result of resonator composed of ceramic            composition out of the scope of this invention.                           

What is claimed is:
 1. A dielectric ceramic composition having in the microwave frequency region a relative dielectric constant of at least 40, an unloaded Q of at least 200 and an absolute value of a temperature coefficient of resonant frequency of at most 150 ppm/°C., and comprising lead oxide, zirconium oxide, and calcium oxide expressed by the formula, xPbO-yZrO₂ -zCaO, wherein x, y, z are in the following mole percentage ranges:0.125≦x≦0.49 0.4≦y≦0.6 0.11≦z≦0.275where x+y+z=1.
 2. The dielectric ceramic composition of claim 1, wherein x, y, z are in the following mole percentage ranges:0.28≦x≦0.425 0.45≦y≦0.52 0.125≦z≦0.2where x+y+z=1.
 3. The dielectric composition of claim 1 wherein the ZrO₂ is partially replaced with (B1mB2n)O₂ resulting in a composition of the formula PB_(1-x) Ca_(x) Zr_(1-y) (B1mB2n)O₂ in which B1 is an element selected from the group consisting of Mg, Ca, Zn, Co, Ni, Mn, Cu, Al, Ga, and Y, B2 is an element selected from the group consisting of Nb, Ta, Sb, and W, x and y are in the mol % ranges of 0.25≦x≦0.5 and 0≦y≦0.2, and m and n are values which satisfy the equation axm+bxn=4, m+n=1, and a is the valence of element B1 and b is the valence of element B2.
 4. The dielectric ceramic composition of claim 3, wherein B1 is Mg, B2 is Nb, and x and y, when expressed in the formula of (Pb_(1-x) Ca_(x))Zr_(1-y) (Mg_(1/3) Nb_(2/3))_(y) O₃, are within a range of a polygon having vertexes at the following points A, B, C, D, E, F, on the xy plane:

    ______________________________________                                                A; x = 0.35   y = 1.0                                                          B; x = 0.6    y = 1.0                                                          C; x = 0.6    y = 0.25                                                         D; x = 0.5    y = 0.0                                                          E; x = 0.25   y = 0.0                                                          F; x = 0.45   y = 0.5.                                                  ______________________________________                                    


5. The dielectric ceramic composition of claim 3, wherein B1 is Ni, B2 is Nb, and x and y, when expressed in the formula of (Pb_(1-x) Ca_(x))Zr_(1-y) (Ni_(1/3) Nb_(2/3))_(y) O₃, are within a range of a quadrangle having vertexes at the following points A, B, C, D on the xy plane:

    ______________________________________                                                A; x = 0.3    y = 0.50                                                         B; x = 0.6    y = 0.5                                                          C; x = 0.3    y = 0.0                                                          D; x = 0.5    y = 0.0.                                                  ______________________________________                                    


6. The dielectric ceramic composition of claim 1, wherein expressing the composition having a part of ZrO₂ of the ceramic composition replaced by SnO₂ as (Pb_(1-x) Ca_(x))Zr_(1-y) Sn_(y) O₃, x and y are within a range of a quadrangle having vertexes at the following points A, B, C, D on the xy plane:

    ______________________________________                                                A; x = 0.3    y = 0.0                                                          B; x = 0.5    y = 0.0                                                          C; x = 0.4    y = 0.6                                                          D; x = 0.25   y = 0.6.                                                  ______________________________________                                    


7. A dielectric resonator comprising a sinter of the dielectric composition of any one of claims 1-6. 